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Silylation and desilylation are important functional group manipulations in solution-phase organic chemistry that are heavily used to protect/deprotect different functionalities. Herein, we disclose the first examples of the σ-bond metathesis of silylated alkynes with aromatic carboxylic acids on the Ag(111) and Au(111) surfaces to give the corresponding terminal alkynes and silyl esters, which is supported by density functional theory calculations and further confirmed by X-ray photoelectron spectroscopy analysis. Such a protecting group strategy applied to on-surface chemistry allows self-assembly structures to be generated from molecules that are inherently unstable in solution and in the solid state. This is shown by the successful formation of self-assembled hexaethynylbenzene at Ag(111). Furthermore, it is also shown that on the Au(111) surface this σ-bond metathesis can be combined with Glaser coupling to fabricate covalent polymers via a cascade process.

Non-thermal plasmas were used as radical initiators in three radical chain reactions: dehalogenation, 5-exo-trig cyclisation and trifluoromethylation with the Togni II reagent. The use of non-volatile solvents (or solvent-free conditions) gave high yields in all reactions under optimised conditions. Short reaction times and mild conditions make this method an attractive alternative to conventional radical initiators.

We present the self-assembly of redox-responsive polymer nanocontainers comprising a cyclodextrin vesicle core and a thin reductively cleavable polymer shell anchored via host-guest recognition on the vesicle surface. The nanocontainers are of uniform size, show high stability, and selectively respond to a mild reductive trigger as revealed by dynamic light scattering, transmission electron microscopy, atomic force microscopy, a quantitative thiol assay, and fluorescence spectroscopy. Live cell imaging experiments demonstrate a specific redox-responsive release and cytoplasmic delivery of encapsulated hydrophilic payloads, such as the pH-probe pyranine, and the fungal toxin phalloidin. Our results show the high potential of these stimulus-responsive nanocontainers for cell biological applications requiring a controlled delivery.

We report on a series of eight diaryl-6-oxo-verdazyl radicals containing a tert-butyl group at the C(3) position with regard to their crystal structure and magnetic properties by means of magnetic susceptibility measurements in combination with quantum chemical calculations using a first-principles bottom-up approach. The latter method allows for a qualitative prediction and detailed analysis of the correlation between the solid-state architecture and magnetic properties. Although the perturbation in the molecular structure by varying the substituent on the N-aryl ring may appear small, the effects upon the structural parameters controlling intermolecular magnetic coupling interactions are strong, resulting in a wide spectrum of cooperative magnetic behavior. The non-substituted 1,5-diphenyl-tert-butyl-6-oxo-verdazyl radical features a ferromagnetic one-dimensional spin ladder type magnetic network-an extremely rarely observed phenomenon for verdazyl radicals. By varying substituents at the phenyl group, different non-isostructural compounds were obtained with widely different magnetic motifs ranging from linear and zigzag one-dimensional chains to potentially two-dimensional networks, from which we predict magnetic susceptibility data that are in qualitative agreement with experiments and reveal a large sensitivity to packing effects of the molecules. The present study advances the fundamental understanding between solid-state structure and magnetism in organically based radical systems.

An efficient method for the asymmetric intramolecular dearomatization of indoles by using oxidative N-heterocyclic carbene catalysis is demonstrated. Valuable optically active spirocyclic indolenines bearing an all-carbon quaternary stereocenter are obtained in excellent yields and with excellent enantioselectivity. The starting indoles are readily prepared and the reactions proceed through an intramolecular indole 3-acylation with an in situ generated acyl azolium intermediate to form a spirocyclic ketone moiety.

A method for regio- and stereoselective anti-addition of the perfluoroalkyl and the triflate group of phenyl(perfluoroalkyl)iodonium triflates to alkynes is presented. The radical reaction uses cheap CuCl as a smart initiator and can be conducted in gram scale. The perfluoroalkyltriflated products are readily further functionalized, rendering this transformation valuable.

Transition metal-free radical α-perfluoroalkylation with concomitant β-alkynylation of unactivated alkenes is presented. These cascades proceed via electron-catalysis and comprise a radical 1,4- or 1,5-alkynyl migration from tertiary propargylic alcoholates to secondary or tertiary C-radicals as the key step. Alkynyl migration leads to a ketyl radical anion that sustains the chain as a single electron transfer reducing reagent.

The dimesitylphosphinocyclopentene/HB(C6 F5 )2 -derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane-borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO2 . It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C6 F5 )2 ] to generate a FLP-η2 -formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.

A practical method for radical chain reduction of various aryl bromides and chlorides is introduced. The thermal process uses NaH and 1,4‐dioxane as reagents and 1,10‐phenanthroline as an initiator. Hydrodehalogenation can be combined with typical cyclization reactions, proving the nature of the radical mechanism. These chain reactions proceed by electron catalysis. DFT calculations and mechanistic studies support the suggested mechanism.

A simple and efficient approach to 6-perfluoroalkylated phenanthridines starting with readily prepared isonitriles and commercially available cheap perfluoroalkyl iodides as perfluoroalkyl radical precursors is described. Various 6-perfluoroalkylated phenanthridines are obtained in moderate to excellent yields. The sequence comprises a perfluoroalkylation with concomitant arene formation.

Die radikalische Carboiodierung verschiedener Arylamine wird vorgestellt. Aryldiazoniumsalze, in situ aus den entsprechenden Arylaminen erzeugt, reagieren dabei mit Bu4NI zu Arylradikalen, welche 5-exo oder 6-exo-Cyclisierungen eingehen. Anschliesende I-Abstraktion liefert die carboiodierten Produkte in guten bis exzellenten Ausbeuten. Bei Zugabe von TEMPO fuhrt die Kaskade zu den entsprechenden carboaminoxylierten Produkten. Lauft die Reaktion in Gegenwart von PhTeTePh ab, so resultieren carbotellurierte cyclische Verbindungen.

This paper describes the synthesis of several highly sterical hindered alkoxyamines based on 6-membered cyclic nitroxides and their application as initiators/regulators in the controlled nitroxide-mediated radical polymerization of n-butyl acrylate and styrene. Rate constant kd and activation energy Ea of the C–O bond of the novel alkoxyamines are reported. Conformational studies on the new nitroxides are performed by X-ray analysis and by DFT calculations. The polymerization of n-butyl acrylate can be conducted at 60 °C showing that these hindered nitroxides are highly valuable to conduct nitroxide-mediated acrylate polymerization (NMP). In addition, low temperature (50 °C) styrene polymerizations are disclosed.

The MgBr2-catalyzed formal [3+2] cycloaddition of donor-acceptor activated cyclopropanes with nitrosoarenes offers a novel approach to various structurally diverse isoxazolidines. The reactions, which are experimentally easy to conduct, occur with complete stereospecificity and perfect control of regioselectivity. Product isoxazolidines can be readily transformed into α-amino lactones by reductive or decarboxylative N-O cleavage and subsequent lactonisation, and the N-aryl bond cleavage is also possible under oxidative conditions.

Functionalization of metal, glass, and semiconductor substrates with polymers and nanoparticles is a key challenge for surface-based material science. Such substrates have the potential to find widespread application in optical and electronic devices, microarrays, and materials for information storage. We report site-specific immobilization of nitroxide-mediated polymerization initiators via microcontact chemistry employing thiol–ene click chemistry on glass-supported alkene-terminated self-assembled monolayers. The polymer initiator covered substrates are used for preparation of poly(styrene) and poly(acrylate) brushes with defined and tunable brush thicknesses. Brush thickness dependent site-specific protein adsorption of streptavidin and concanavalin A on structured polystyrene brushes is reported. Poly(styrene) brushes with a thickness of 40 nm or larger showed protein repellence whereas brushes below 15 nm thickness reveal protein adhesive properties. We also disclose the site selective host–guest as...

On-surface synthesis is a powerful cutting-edge technology to build up covalently bound nanostructures directly at surfaces which enables the preparation of highly advanced one- or two-dimensional materials. In these processes, bond formation generally occurs by thermal activation of the precursor building blocks. In this Article we report the light-induced homocoupling of aryl-alkynes (Glaser coupling) at metal surfaces. Such photochemical approaches are of particular importance as potentially orthogonal processes to thermal on-surface reactions.

Copper-catalyzed intermolecular aminoazidation of alkenes is described. This novel methodology provides an efficient approach to vicinal amino azides which can easily be transformed into other valuable amine derivatives. The commercially available N-fluorobenzenesulfonimide (NFSI) is used as a nitrogen-radical precursor and TMSN3 as the N3 source. Yields are moderate to excellent, and for internal alkenes aminoazidation occurs with excellent diastereoselectivity.

Ein neuartiges modulares Konzept fur den kontrollierten Zugang zu racemischen Resveratrol-basierten Naturstoffen wird beschrieben. Ausgehend von nur einem leicht zuganglichen Baustein konnen durch palladiumkatalysierte decarboxylierende Arylierung und oxidative Heck-Reaktion strukturell entscheidende Arylreste variabel eingefuhrt werden. Die Schlusselschritte laufen dabei in guter Ausbeute und mit hoher Selektivitat ab. Das modulare Konzept wird am Beispiel der Totalsynthese von drei Resveratrol-basierten Naturstoffen (Quadrangularin A, Ampelopsin D und Pallidol) belegt.

The application of chromophoric radical spin traps for highly sensitive UV–vis monitoring of conversion of radical enhanced spin capturing polymerizations (ESCP) is presented. In ESCP the growing macroradical is reacting with a nitrone providing the corresponding polymeric nitroxide radical spin trap which will react irreversibly with a second macroradical to eventually give a polymeric alkoxyamine. The progress of styrene and n-butyl acrylate ESC polymerizations in the presence of π-conjugated nitrones is successfully visualized via a significant change of absorbance of the nitrone chromophore in the 200–500 nm range due to the nitrone transformation into an alkoxyamine moiety upon incorporation into the polymer. Moreover, related studies on UV–vis monitoring of photochemical rearrangement of nitrones to their corresponding oxaziridines are discussed.

A simple and efficient approach to biologically important 1-trifluoromethylated isoquinolines starting with readily prepared β-aryl-α-isocyano-acrylates and the commercially available Togni reagent as the CF3 radical precursor is described. These transformations occur in the absence of any transition metals and the title compounds are obtained in moderate to excellent yields. This protocol comprises a trifluoromethylation with concomitant isoquinoline framework construction.

N-heterocyclic carbene-catalyzed oxidative esterification of various aliphatic aldehydes is reported. A commercially available bisquinone is used as an external oxidant and rubidium carbonate is best suited as base to run these processes. With the novel method various aliphatic esters are directly obtained in good to excellent yields from the corresponding aldehydes and alcohols.

Bioorthogonal ligation methods are the focus of current research due to their versatile applications in biotechnology and materials science for post-functionalization and immobilization of biomolecules. Recently, inverse electron demand Diels-Alder (iEDDA) reactions employing 1,2,4,5-tetrazines as electron deficient dienes emerged as powerful tools in this field. We adapted iEDDA in microcontact chemistry (μCC) in order to create enhanced surface functions. μCC is a straightforward soft-lithography technique which enables fast and large area patterning with high pattern resolutions. In this work, tetrazine functionalized surfaces were reacted with carbohydrates conjugated with norbornene or cyclooctyne acting as strained electron rich dienophiles employing μCC. It was possible to create monofunctional as well as bifunctional substrates which were specifically addressable by proteins. Furthermore we structured glass supported alkene terminated self-assembled monolayers with a tetrazine conjugated atom transfer radical polymerization (ATRP) initiator enabling surface grafted polymerizations of poly(methylacrylate) brushes. The success of the surface initiated iEDDA via μCC as well as the functionalization with natural and synthetic polymers was verified via fluorescence and optical microscopy, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR).

Possible origins of the formation of organometallic intermediates in on-surface Ullmann couplings have been investigated by surface tunneling microscopy (STM) and density functional theory (DFT) calculations. We consider the case of iodobenzenes on the coinage metals copper, silver, and gold. We found experimental evidence for the formation of surface vacancies and the presence of metal adatoms in these coupling reactions, which are taken as a hint for the reactive extraction of surface atoms by adsorbates. In a second step, we demonstrate by ab initio molecular dynamics calculations for aryl-iodides on copper that metal atoms can be pulled out of the surface to form metal-organic species. By contrast, a thermally activated provision of a metal atom from the surface to form an adatom is energetically unfavorable. Finally, we investigate the mechanism and energetics of the reactive extraction of surface metal atoms by means of (climbing-image) nudged elastic band density-functional theory calculations for iodobenzene on copper, silver, and gold, and analyze our results in the light of the experimental findings.

New P/N-containing π-electron systems comprising fully planar biaryl arrays are synthesized by multiple radical phosphanylation. The biaryl moiety in these highly strained planar π-systems is rigidified by double P-bridging. The electronic properties of the core biaryl entity are varied by introducing N-donor substituents or by installing N-atoms within the π-system, thereby moving to the viologen core structure. The electrochemical and photophysical properties of these compounds are discussed and compared with those of related systems.

Moreover, the corresponding tetrasubstituted alkenes are obtained with complete E-selectivity.

A C-P bond and a C-C bond are formed in the synthesis of 6-phosphorylated phenanthridines starting with readily prepared 2-isocyanobiphenyls and commercially available P-radical precursors. The radical cascade reaction comprises addition of an oxidatively generated P-centered radical to the isonitrile functionality and subsequent homolytic aromatic substitution. Various 6-phosphorylated phenanthridines are formed in moderate to excellent yield. In contrast to the currently intensively investigated direct arene phosphorylation, the arene core is constructed with concomitant phosphorylation using this approach.

Composite nanofibers of polyurethane and catalytically active Pd- or Cu-functionalized poly(amido amine) dendrimers were electrospun and subsequently coated with poly(p-xylylene) by the chemical vapor deposition. After removal of the inner polyurethane template matrix, the dendrimers remained encapsulated inside the confinement and the leaching behavior of the caged dendritic catalysts was studied. The tube system was successfully used as a readily reusable tea bag type catalyst for Suzuki–Miyaura couplings and alkyne/azide click reactions.

A transition-metal-free phenanthrene synthesis starting from readily accessible ortho-amino-biaryls is presented. The biarylamines are in situ transformed into the corresponding diazonium salts which upon single electron reduction give the corresponding aryl radicals. Addition to an alkyne and subsequent base promoted homolytic aromatic substitution (BHAS) provide phenanthrenes in moderate to good yields.

We present [3 + 2] cycloaddition reactions between azides and alkynes on a Au(111) surface at room temperature and under ultrahigh vacuum conditions. High-resolution scanning tunneling microscopy images reveal that these on-surface cycloadditions occur highly regioselectively to form the corresponding 1,4-triazoles. Density functional theory simulations confirm that the reactions can occur at room temperature, where the Au(111) surface does not participate as a catalytic agent in alkyne C-H activation but acts solely as a two-dimensional constraint for the positioning of the two reaction partners. The on-surface azide-alkyne cycloaddition offers great potential toward the development and fabrication of functional organic nanomaterials on surfaces.

The homocoupling of alkynes at metal surfaces, which was disclosed recently, is a promising reaction for efficient construction of conjugated nanostructures at metal surfaces. However, the role of the metal substrate as well as the mechanistic course of this process have not been investigated. The metal surface could act cooperatively (a) for two-dimensional confinement to properly orient the organic reactant and (b) also as an active mediator in the C–C bond-forming reaction. Herein we report covalent coupling of the dimers of 1,4-diethynylbenzene at various metal surfaces. The model reaction was investigated experimentally by STM and also by theoretical DFT calculations. Detailed statistical analysis and the theoretical results strongly support the involvement of the metal surface in the C–C bond-forming process. On the basis of these investigations, a model with two possible reaction pathways is suggested to describe the process: C–C coupling via direct CH activation and C–C coupling via alkynyl activa...

Radical azidooxygenation of various alkenes is described. A readily prepared N3-iodine(III) reagent acts as a clean N3-radical precursor. Radical generation is achieved with TEMPONa acting as a mild organic reducing reagent. The C-radical generated after N3-radical addition is efficiently trapped by in situ generated TEMPO. Yields are good to excellent, and for cyclic systems azidooxygenation occurs with excellent diastereoselectivity.

A bifunctional heterogeneous palladium catalyst bearing additional basic sites was successfully prepared by sequential Cu-catalyzed 1,3-dipolar alkyne–azide cycloaddition and thermal nitroxide-exchange reaction of surface-bound alkoxyamines. This catalyst shows high activity in the Suzuki–Miyaura cross-coupling. The additional basic functionality acts cooperatively since an analogous heterogeneous Pd catalyst lacking the amine functionality is less active. Such catalysts can be recycled up to ten times without loss of activity.

This article describes N-heterocyclic carbene catalyzed transesterification of polytrifluoroethyl acrylates which are readily prepared by nitroxide-mediated radical polymerization using a sterically hindered alkoxyamine as an initiator/regulator. Controlled radical polymerization provides polytrifluoroethyl acrylates with molecular weights (Mn) ranging from 1600 to 17200 g/mol and low molecular weight distributions (polydispersity index, PDI = 1.1–1.2). The polytrifluoroethyl acrylates are readily transesterified with various alcohols in the presence of an NHC catalyst.

The preparation of polymer brushes (PBs) bearing α-hydroxyalkylphenylketone (2-hydroxy-2-methyl-1-phenylpropan-1-one) moieties as photoreactive polymer backbone substituents is presented. Photoreactive polymer brushes with defined thicknesses (up to 60 nm) and high grafting densities are readily prepared by surface initiated nitroxide mediated radical polymerization (SINMP). The photoactive moieties can be transformed via Norrish-type I photoreaction to surface-bound acyl radicals. Photolysis in the presence of a persistent nitroxide leads to chemically modified PBs bearing acylalkoxyamine moieties as side chains resulting from trapping of the photogenerated acyl radicals with nitroxides. Application of functionalized nitroxides to the photochemical PB postmodification provides functionalized PBs bearing cyano, polyethylene glycol (PEG), perfluoroalkyl, and biotin moieties. As shown for one case, photochemical postfunctionalization of the PB through a mask using a biotin-conjugated nitroxide as the trapping reagent leads to the corresponding site-selective chemically modified PB, which is successfully used for site-specific streptavidin immobilization. Surface analysis of PBs was performed by contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), attenuated total reflection (ATR), fourier transform infrared (FTIR) spectroscopy, and fluorescence microscopy.

The paper describes a cooperative NHC (N-heterocyclic carbene) and ruthenium-based redox catalysis for the mild aerobic oxidative esterification of various aromatic and heteroaromatic aldehydes. The ruthenium(II) complex Ru(bpz)3(PF6)2 (bpz=2,2′-bipyrazine) as catalyst is shown to be compatible with free NHCs. The NHC is used in these cascade reactions for the umpolung of the aldehyde to form the corresponding Breslow intermediate which in turn gets oxidized to an acylazolium ion by the ruthenium redox catalyst. Air is used as a terminal oxidant for oxidation (regeneration) of the ruthenium catalyst. In addition, we will show that in the absence of the ruthenium redox catalyst and alcohol, NHC-catalyzed aerobic oxidation of aldehydes delivers the corresponding acids in good to excellent yields. Mechanistic studies and DFT calculations supporting our suggested mechanisms are provided.

A novel synthetic route toward poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (PTMA) is described. The polymerization of alkoxyamine-based monomers by atom transfer radical polymerization (ATRP) was investigated, as the polyalkoxyamine serves as the precursor for PTMA. The polydispersity indices (PDIs) and the kinetic data of the polymerization indicate a controlled reaction. The oxidative C–O bond cleavages of the polyalkoxyamine lead to PTMA. This transformation occurs with excellent yields, and it is possible to transfer the narrow PDIs of the prepolymer to PTMA. The material is characterized in detail using cyclic voltammetry in solution and magnetic susceptibility measurements as well as multinuclear solid state NMR and EPR spectroscopies. The conversion of the precursor polymer to the polynitroxide can be conveniently monitored by 1H and 19F magic-angle spinning (MAS) as well as 13C{1H} cross-polarization (CP)-MAS NMR. In addition, the intermolecular interaction of the nitroxide side c...

This Concept article discusses the potential of oxidative carbene catalysis in synthesis and comprehensively covers pioneering studies as well as recent developments. Oxidative carbene catalysis can be conducted by using inorganic and organic oxidants. Applications in cascade processes, in enantioselective catalysis, and also in natural product synthesis are discussed.

Cross dehydrogenative coupling reactions occurring via base-promoted homolytic aromatic substitutions (BHASs) are reported. Fluorenones and xanthones are readily prepared via CDC starting with readily available ortho-formyl biphenyls and ortho-formyl biphenylethers, respectively. The commercially available and cheap tBuOOH is used as an oxidant. Initiation of the radical chain reaction is best achieved with small amounts of FeCp(2) (0.1 or 1 mol %).

Cooperative effects can be observed in various research areas in chemistry; cooperative catalysis is well-established, the assembly of compounds on surfaces can be steered by cooperative effects, and supramolecular polymerization can proceed in a cooperative manner. In biological systems, cooperativity is observed in protein-protein, protein-lipid and protein-molecule interactions. Synergistic effects are relevant in frustrated Lewis pairs, organic multispin systems, multimetallic clusters and also in nanoparticles. However, a general approach to determine cooperativity in the different chemical systems is currently not known. In the present concept paper it is suggested that, at least for simpler systems that can be described at the molecular level, cooperativity can be defined based on energy considerations. For systems in which no chemical transformation occurs, determination of interaction energies of the whole system with respect to the interaction energies between all individual component pairs (subsystems) will allow determination of cooperativity. For systems comprising of chemical transformations, cooperativity can be evaluated by determining the activation energy of the synergistic system and by comparing this with activation energies of the corresponding subsystems that lack an activating moiety. For more complex systems, cooperativity is generally determined at a qualitative level.